Electrolytic solution for use in electrolytic capacitor and electrolytic capacitor

ABSTRACT

An electrolytic solution for use in an electrolytic capacitor, having a solution containing a solvent consisting of 20 to 80% by weight of an organic solvent and 80 to 20% by weight of water, and at least one electrolyte selected from a carboxylic acid or a salt thereof and an inorganic acid or a salt thereof, having added thereto at least on nitro compound selected from nitrophenol, nitrobenzoic acid, dinitrobenzoic acid, nitroacetophenone and nitroanisole. The electrolytic solution has a low impedance and excellent low-temperature stability, along with good working life characteristics, and it can also exhibit an excellent hydrogen gas absorption function when an electrolytic solution contains a highly increased amount of water in its mixed solvent or when an electrolytic capacitor is used under high temperature conditions.

TECHNICAL FILED

The present invention relates to an electrolytic capacitor. Moreparticularly, the present invention relates to an electrolytic solutionfor use in an electrolytic capacitor, which has a low impedance andexcellent low-temperature stability, along with good characteristics ofworking life, and an electrolytic capacitor using the same, specially analuminum electrolytic capacitor.

BACKGROUND ART

Generally, a capacitor is a general electrical part and is widely usedfor as a power supply circuit, a noise filter and a digital circuitcomponent in various electric/electronic parts. Capacitors are roughlyclassified into electrolytic capacitors and other capacitors such asceramic capacitors, film capacitors, etc.

Various types of electrolytic capacitors are used at present andexamples thereof include aluminum electrolytic capacitors, wet tantalumelectrolytic capacitors and the like. It is an aluminum electrolyticcapacitor from which a particularly excellent function is expected inthe present invention. Therefore, the present invention will now bedescribed with reference to this kind of an electrolytic capacitor. Theterm “electrolytic capacitor” used herein refers to an aluminumelectrolytic capacitor unless otherwise stated.

A conventional aluminum electrolytic capacitor can be produced typicallyby using an anode foil, which is made by etching a high-purity aluminumfoil to thereby increase its surface area, and anodizing the surface ofthe aluminum foil to provide an oxide film, and a cathode foil whosesurface has only been etched. The resulting anode foil and cathode foilare disposed opposite each other and an element with a wound structureis made by interposing a separator (release paper) between those foilsand then the element is impregnated with an electrolytic solution. Theelement impregnated with the electrolytic solution is contained in acase (generally made of aluminum), which is then sealed with an elasticsealant, thus completing an electrolytic capacitor. Electrolyticcapacitors also include electrolytic capacitors other than those with awound structure.

In the above-described electrolytic capacitor, the characteristics ofthe electrolytic solution may be a large factor which decides theperformance of the electrolytic capacitor. With the size reduction ofthe electrolytic capacitor, an anode foil or cathode foil having a largesurface area produced by etching has been used and the resistivity ofthe capacitor has recently increased. Therefore, an electrolyticsolution having a low resistivity (specific resistance) and thus highconductivity is required as an electrolytic solution to be used in theelectrolytic capacitor.

A conventional electrolytic solution for use in an electrolyticcapacitor is generally prepared by dissolving, as an electrolyte, acarboxylic acid such as adipic acid, benzoic acid, etc. or an ammoniumsalt thereof into a solvent prepared by adding about 10% by weight orless of water to ethylene glycol (EG) as a principal solvent. Such anelectrolytic solution has a specific resistance of about 1.5 Ω·m (150Ω·cm).

On the other hand, the capacitor is required to have a low impedance (Z)to sufficiently exert the performance thereof. The impedance is decidedby various factors and, for example, it is reduced when the electrodearea of the capacitor increases. Therefore, an attempt to reduce theimpedance is made as a matter of course in case of a large-sizedcapacitor. An attempt to reduce the impedance by improving a separatorhas also been made. However, the specific resistance of the electrolyticsolution is a large controlling factor, particularly in a small-sizedcapacitor.

A lower-specific resistance electrolytic solution using an aproticorganic solvent such as GBL (γ-butyrolactone) has recently beendeveloped (see, Japanese Unexamined Patent Publication (Kokai) Nos.62-145713, 62-145714 and 62-145715). However, the capacitor using thisaprotic electrolytic solution is by far inferior in impedance to a solidcapacitor using an electronic conductor having a specific resistance of1.0 Ω·cm or less.

The aluminum electrolytic capacitor has poor low-temperature stabilitybecause of use of an electrolytic solution, and a ratio of an impedanceat −40° C. to that at 20° C. (100 kHz), Z (−40° C.)/Z (20° C.), is aslarge as about 40 at present. Under these circumstances, it is nowrequired to provide an aluminum electrolytic capacitor which has a lowimpedance and excellent low-temperature stability.

Further, water used as portion of the solvent in the electrolyticsolution of the aluminum electrolytic capacitor is a chemically activesubstance to aluminum constituting the anode foil or cathode foil.Accordingly, there is a problem that water reacts with the anode foil orcathode foil, thereby to generate a hydrogen gas and to drasticallydeteriorate the performance as a capacitor.

To solve a problem such as generation of hydrogen gas found in a loadlife test of the electrolytic capacitor, a trial of absorbing thegenerated hydrogen gas has hitherto been made. For example, JapaneseExamined Patent Publication (Kokoku) No. 59-15374 discloses anelectrolytic solution, for use in operation of an electrolyticcapacitor, produced by adding a carboxylic acid and an ammonium salt ofthe carboxylic acid to a solvent having added thereto 5 to 20% by weightof water, thereby to prepare a buffer solution and further adding 0.05to 3% by weight of p-nitrophenol to the buffer solution. When using thiselectrolytic solution, there can be provided an electrolytic capacitorwherein low-temperature stability and a working life characteristics areimproved by inhibiting the occurrence of the boehmite reaction andgeneration of the hydrogen gas.

Japanese Unexamined Patent Publication (Kokai) No. 63-14862 alsodiscloses an electrolytic solution for use in the operation of anelectrolytic capacitor capable of exhibiting an excellent corrosionpreventing function against washing with a halogenated hydrocarbon,which is produced by adding o-nitroanisole to an electrolytic solutionprepared by dissolving various organic acids, inorganic acids or saltsthereof in a solvent composed exclusively of ethylene glycol. Thispublication describes that o-nitroanisole used as a corrosion inhibitorhas a hydrogen gas absorption function, that is, a function of absorbinga hydrogen gas generated from the interior during the use of theelectrolytic capacitor, thereby making it possible to inhibit anaccident of safety-vent operation and a change in capacitance.

However, it has been found, as a result of the present inventors' study,that p-nitrophenol or o-nitroanisole can exhibit an initial hydrogenabsorption function in the case of a conventionally used electrolyticsolution of low water concentration for use in operation of anelectrolytic capacitor, but cannot exhibit and maintain a satisfactoryhydrogen gas absorption function when a content of water is 20% byweight or more based on the solvent in the electrolytic solution or whenthe electrolytic capacitor is operated under high temperature conditionsfor a long period of time.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished to solve the above problemsof the prior art, and an object thereof is to provide an electrolyticsolution, for use in an electrolytic capacitor, which has a lowimpedance and excellent low-temperature stability, expressed by an aratio of an impedance at low temperature to that at normal temperature,along with good characteristics of working life, and also it can exhibitan excellent hydrogen gas absorption function even when an electrolyticsolution contains a highly increase amount of water in its mixed solventor when an electrolytic capacitor is used under high temperatureconditions.

Another object of the present invention is to provide an electrolyticcapacitor using the electrolytic solution of the present invention,specially an aluminum electrolytic capacitor.

These objects as well as other objects of the present invention willeasily become apparent from the following detailed description.

In one aspect thereof, the present invention resides in an electrolyticsolution for use in an electrolytic capacitor, comprising a solutioncontaining a solvent consisting of 20 to 80% by weight of an organicsolvent and 80 to 20% by weight of water, and at least one electrolyteselected from the group consisting of a carboxylic acid or a saltthereof and an inorganic acid or a salt thereof, having added thereto atleast one nitro compound selected from the group consisting ofnitrophenol, nitrobenzoic acid, dinitrobenzoic acid, nitroacetophenoneand nitroanisole.

In the electrolytic solution of the present invention, the nitrocompound can exhibit an excellent hydrogen absorption function incombination with the other electrolytic solution component on even whenthe nitro compound is used alone. To obtain a more remarkable function,two or more nitro compounds are used in combination, more preferably.

When the nitro compound is added to the electrolytic solution of thepresent invention, the nitro compound is added in the amount of 0.01 to5% by weight based on the total amount of the electrolytic solution.

The organic solvent to be used, along with water, to form a mixedsolvent is a protic solvent, an aprotic solvent, or a mixture thereof.That is, the protic solvents and aprotic solvents may be used alone ortwo or more kinds of them may be optionally used in combination,respectively. The protic solvent is preferably an alcohol compound,while the aprotic solvent is preferably a lactone compound.

The carboxylic acid or salt thereof to be used as the electrolyte in theelectrolytic solution of the present invention is preferably at leastone selected from the group consisting of formic acid, acetic acid,propionic acid, butyric acid, p-nitrobenzoic acid, salicylic acid,benzoic acid, oxalic acid, malonic acid, succinic acid, glutaric acid,adipic acid, fumaric acid, maleic acid, phthalic acid, azelaic acid,citric acid and hydroxybutyric acid, and ammonium, sodium, potassium,amine and alkyl ammonium salts thereof.

The inorganic acid or salt thereof which is also used as the electrolyteis at least one selected from the group consisting of phosphoric acid,phosophorous acid, hydrophosphorous acid, boric acid, sulfamic acid, andammonium, sodium, potassium, amine and alkyl ammonium salts thereof.

In addition to the nitro compound, additives selected from the groupconsisting of the following group:

(1) a chelate compound,

(2) saccharides,

(3) hydroxybenzyl alcohol and/or L-glutamic-diacetic acid or a saltthereof, and

(4) gluconic acid and/or gluconic lactone may be optionally contained inthe electrolyte of the present invention. These additives may be usedalone, or two or more kinds of them may be optionally used incombination.

In another aspect thereof, the present invention resides in anelectrolytic capacitor comprising an electrolytic solution for use in anelectrolytic capacitor which comprises a solution containing a solventconsisting of 20 to 80% by weight of an organic solvent and 80 to 20% byweight of water, and at least one electrolyte selected from the groupconsisting of a carboxylic acid or a salt thereof and an inorganic acidor a salt thereof, having added thereto at least one nitro compoundselected from the group consisting of nitrophenol, nitrobenzoic acid,dinitrobenzoic acid, nitroacetophenone and nitroanisole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing one preferred embodiment of theelectrolytic capacitor according to the present invention, and

FIG. 2 is a perspective view showing the constitution of a capacitorelement of the electrolytic capacitor shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, the electrolytic solution for an electrolyticcapacitor according to the present invention is characterized bycontaining:

at least one nitro compound selected from the group consisting ofnitrophenol, nitrobenzoic acid, dinitrobenzoic acid, nitroacetophenoneand nitroanisole,

in addition to a solvent consisting of 20 to 80% by weight of an organicsolvent and 80 to 20% by weight of water, and

at least one electrolyte selected from the group consisting of acarboxylic acid or a salt thereof and an inorganic acid or a saltthereof.

In the electrolytic solution for an electrolytic capacitor according tothe present invention, a solvent containing a highly increased amount ofwater, which consists of a mixture of an organic solvent and water, isused as the solvent for dissolving the electrolyte.

As described above, protic solvents or aprotic solvents are used aloneor, optionally, in combination. Examples of preferred protic solventinclude alcohol compound. Specific examples of the alcohol compound usedadvantageously herein include, but are not limited to, monohydricalcohol such as ethyl alcohol, propyl alcohol, and butyl alcohol;dihydric alcohol such as ethylene glycol, diethylene glycol, triethyleneglycol, and propylene glycol; and trihydric alcohol such as glycerin.Examples of preferred aprotic solvent include lactone compounds.Specific examples of the lactone compounds used advantageously hereininclude, but are not limited to, γ-butyrolactone and otherintramolecular polarizable compounds. When using at least one solventselected from the protic and aprotic solvents in the practice of thepresent invention, more specifically, one protic solvent may be used,one aprotic solvent may be used, plural protic solvents may be used,plural aprotic solvents may be used, alternatively a mixed solvent of atleast one protic solvent and at least one aprotic solvent may be used.

In the electrolytic solution of the present invention, water is added inaddition to the above-described organic solvents as the solventcomponent. Particularly, the present invention differs from aconventional electrolytic solution in that a comparatively large amountof water is used. According to the present invention, by using such asolvent, the solidifying point of the solvent is lowered, thereby makingit possible to improve the specific resistance at low temperature of theelectrolytic solution and to realize good low-temperature stability,expressed by a ratio of a resistivity at low temperature to that atnormal temperature. A content of water in the electrolytic solution ispreferably within a range from 20 to 80% by weight, and an organicsolvent is contained as a balance. When the content of water is smallerthan 20% by weight and when the content of water exceeds 80% by weight,the degree of depression in solidifying point of the electrolyticsolution becomes insufficient, thereby making it difficult to obtaingood low-temperature stability of the electrolytic capacitor. Apreferred content of water in the solvent is within a range from 30 to80% by weight, and a most preferred content of water in the solvent iswithin a range from 45 to 80% by weight.

As the electrolyte in the electrolytic solution of the presentinvention, an organic acid, particularly preferably a carboxylic acid ora salt thereof, and an inorganic acid or a salt thereof may be used.These electrolyte components may be used alone, or two or more kinds ofthem may be used in combination.

Examples of carboxylic acid which can be used as the electrolytecomponent include, but are not limited to, monocarboxylic acid such asformic acid, acetic acid, propionic acid, butyric acid, p-nitrobenzoicacid, salicylic acid, and benzoic acid; and dicarboxylic acid such asoxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,fumaric acid, maleic acid, phthalic acid, and azelaic acid. Carboxylicacids having a functional group such as hydroxyl group, for example,citric acid and hydroxybutyric acid may also be used.

Examples of the inorganic acid which can also be used as the electrolytecomponent include, but are not limited to, phosphoric acid, phosophorousacid, hypophosphorous acid, boric acid and sulfamic acid.

As the salt of the above-described carboxylic acid or inorganic acid,various salts can be used. Preferred salts include, for example,ammonium salts, sodium salts, potassium salts, amine salts and alkylammonium salts. Among these salts, an ammonium salt is preferably used.

In addition, using the inorganic acid or salt thereof as the electrolytein the practice of the present invention, depression in solidifyingpoint of the electrolytic solution can be expected, thereby making itpossible to contribute to a further improvement in low-temperaturestability of the electrolytic solution. The use of the inorganic acid orsalt thereof is noticeable in that the hydrogen gas absorbability(described in detail hereinafter) derived from the nitro compound usedparticularly in the present invention can be maintained for a longperiod of time.

According to the present inventors study, by using an electrolyte suchas inorganic acid or salt thereof in combination with theabove-described carboxylic acid or salt thereof, an effect of remarkablyprolonging a working life of the electrolytic capacitor, occurs, ascompared with the case where they are used alone. Furthermore, aninorganic acid-based electrolyte has hitherto been used exclusively in amedium to high-voltage (160 to 500 volts) type electrolytic capacitor ina conventional electrolytic capacitor in view of the conductivity.However, when using electrolytes in combination, like the presentinvention, the electrolyte can also be used advantageously in alow-voltage (lower than 160 volt) type electrolytic capacitor.

The amount of the electrolyte used in the electrolytic solution of thepresent invention can be appropriately determined depending on variousfactors such as characteristics required to the electrolytic solutionand the capacitor obtained finally, kind, compositions and amount of thesolvent, and kind of the electrolyte. As described above, when using theinorganic acid-based electrolyte in combination with the carboxylicacid-based electrolyte, the amount of the inorganic acid-basedelectrolyte contained in the mixed electrolyte can vary within a widerange, but the inorganic acid-based electrolyte is preferably containedin the amount within a range from about 0.1 to 15%, by weight, based onthe total amount of the electrolyte.

The electrolytic solution of the present invention is characterized byfurther adding, as an additive, at least one nitro compound selectedfrom the group consisting of a nitrophenol such as p-nitrophenol, anitrobenzoic acid such as p-nitrobenzoic acid, a dinitrobenzoic acid, anitroacetophenone such as p-nitroacetophenone and nitroanisole, to anelectrolytic solution of the above-described specific compositions, thatis, an electrolytic solution comprising an aqueous mixed solventconsisting of 20 to 80% by weight of an organic solvent and 80 to 20% byweight of water, and at least one electrolyte selected from the groupconsisting of a carboxylic acid or a salt thereof and an inorganic acidor a salt thereof.

In the present invention, a particularly hydrogen gas absorptionfunction could be confirmed when using the above-described group ofnitro compounds, but the actual reasons have not yet been clarified.However, it is considered, based on the present inventors' experience,that a large factor is in that substituents contained in each nitrocompound exhibit the hydrogen gas absorption function at differenttimings. The nitro compound used herein can also have a function ofinhibiting corrosion of the element caused by a function of ahalogenated hydrocarbon used on washing of a printed circuit board, forexample, trichloroethane (a halogen capturing function, in other words).

When the nitro compound is added to the electrolytic solution of thepresent invention, the nitro compound can exhibit satisfactory hydrogengas absorption functions and halogen capturing functions even when usedalone because specific compositions effective for the function of thepresent invention are employed in the electrolytic solution itself.According to the present inventors' finding, a more preferred functioncan be expected from using two or more nitro compounds in combination.It is generally recommended to use two nitro compounds in combination.The nitro compound is preferably added in the amount within a range from0.01 to 5% by weight based on the total amount of the electrolyticsolution. When the amount of the nitro compound is smaller than 0.01% byweight, an expected function is hardly obtained. On the other hand, evenwhen the amount exceeds 5% by weight, a further improvement in expectedfunction cannot be expected and a deleterious influence is sometimesexerted on the other characteristics.

The use of the nitro compound will be described in more detail below.The absorption function of the hydrogen gas generated on the reactionbetween aluminum and water is liable to be lowered with the increase inamount of water in the solvent when using the nitro compound alone, aswas described in the item entitled “Background Art”. This loweringtendency becomes drastic in the case where the electrolytic solution issubjected to high temperature conditions. However, problems caused byusing these nitro compounds alone can be solved by using two or morenitro compounds in combination, as in the present invention. Actually,in case of the electrolytic solution of the present invention, thehydrogen gas absorbability can be maintained under high temperatureconditions for a longer period of time, than in the case where thesenitro compounds are used alone, by using plural nitro compounds.

An excellent function in absorption of the hydrogen gas according to thepresent invention could also be confirmed in a relation to theelectrolyte used in combination. In a conventional electrolyticsolution, the procedure of adding only one nitro compound to only acarboxylic acid-based electrolyte, or adding only one nitro compound toonly an inorganic acid-based electrolyte has been employed. However, asatisfactory hydrogen gas absorption function cannot obtained by theprocedure in case where the amount of water contained in the solvent islarge, and the same results are obtained in an electrolytic solutionwherein both of a carboxylic acid-based electrolyte and an inorganicacid-based electrolyte are present. In case of the electrolytic solutionof the present invention (using only one nitro compound), the hydrogengas absorbability could be, surprisingly, maintained for a longer periodof time than the case where nitro compounds are used alone even in caseof the carboxylic acid/inorganic acid mixed electrolytic solution.

The electrolytic solution of the present invention can optionallycontain, as an additive, components other than those described above.Preferred additives include, for example, the following compounds.

(1) Chelate compound, for example, ethylenediamine-N,N,N′,N′-tetraaceticacid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid,monohydrate (CyDTA), N,N-bis(2-hydroxyethyl)glycine (DHEG),ethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid) (EDTPO),diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA),1,3-diamino-2-hydroxypropane-N,N,N′,N′-tetraacetic acid (DPTA-OH),ethylnediamine-N,N′-diacetic acid (EDDA),ethylenediamine-N,N′-bis(methylenephosphonic acid), hemihydrate (EDDPO),O,O′-bis(2-aminoethyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid (GEDTA),N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (EDTA-OH) andothers. The chelate compound is preferably added in the amount within arange from 0.01 to 3% by weight. Such a chelate compound can exerteffects such as prolongation of a working life due to inhibition of thehydration reaction of an aluminum (Al) electrode foil of a low-impedancecapacitor, improvement in low-temperature stability of an electrolyticcapacitor (a change between an impedance at normal temperature and thatat low temperature decreases because the solvent has compositions closeto those corresponding to a non-frozen state), and improvement incorrosion resistance.

(2) Saccharides, for example, glucose, fructose, xylose, galactose andothers. The saccharides are preferably added in the amount within arange from 0.01 to 5% by weight. These saccharides can exert effectssuch as prolongation of a working life due to inhibition of thehydration reaction of an aluminum electrode foil of a low-impedancecapacitor, inhibition of decomposition or activation of an electrolyte(e.g. carboxylic acid) due to the addition of saccharides, andimprovement in low-temperature stability of an electrolytic capacitor (achange between an impedance at normal temperature and that at lowtemperature decreases because the solvent has compositions close tothose corresponding to a non-frozen state).

(3) Hydroxybenzyl alcohol, for example, 2-hydroxybenzyl alcohol,L-glutamic-diacetic acid or a salt thereof and others. This additive ispreferably added in the amount within a range from 0.01 to 5% by weight.Such an additive can exert effects such as prolongation of a workinglife due to inhibition of the hydration reaction of an aluminumelectrode foil of a low-impedance capacitor, and improvement inlow-temperature stability of an electrolytic capacitor (a change betweenan impedance at normal temperature and that at low temperature decreasesbecause the solvent has compositions close to those corresponding to anon-frozen state).

The above-described compounds (1) to (3) can exhibit various remarkableeffects by adding them to the electrolytic solution of the presentinvention, and almost all of the effects can be expected even in casewhere no nitro compound is contained in the electrolytic solution.According to the present inventors' study, these excellent effects canbe obtained by using at least one of the above compounds (1) to (3) incombination with gluconic acid or gluconic lactone described below.

In addition to the above-described additives (also including the casewhere nitro compounds are added alone), the electrolytic solution of thepresent invention can optionally contain:

(4) gluconic acid and gluconic lactone alone or in combination. Thiskind of the additive is preferably added in the amount within a rangefrom 0.01 to 5% by weight. Gluconic acid and gluconic lactone canfurther exert remarkable effects such as improvement in corrosionresistance, in addition to functions, which are specific to the presentinvention, such as prolongation of a working life of an electrolyticcapacitor, improvement in low-temperature stability and excellenthydrogen gas absorption function, by containing them in the electrolyticsolution of the present invention.

In addition to the above-described additives, additives conventionallyused in the field of aluminum electrolytic capacitors and otherelectrolytic capacitors may also be added. Preferred conventionaladditives include, for example, mannitol, a silane coupling agent, awater-soluble silicone and a polyelectrolyte.

The electrolytic solution of the present invention can be prepared bymixing and dissolving the above-described various components in anarbitrary sequence according to a conventional procedure or a modifiedprocedure thereof. For example, the electrolytic solution can be simplyprepared by preparing a solvent containing a highly increased amount ofwater as a mixture of an organic solvent and water, and optionallydissolving an electrolyte, a nitro compound and optional additives inthe resulting solvent.

According to the present invention, there is also provided anelectrolytic capacitor, preferably an electrolytic capacitor comprisinga capacitor element formed of an anode foil, a cathode foil opposed tothe anode foil and a separator disposed between the anode foil and thecathode foil, and the electrolytic solution of the present invention.

The electrolytic capacitor of the present invention is more preferablyan aluminum electrolytic capacitor, and most preferably an electrolyticcapacitor comprising:

a capacitor element formed by winding an anode foil consisting of analuminum foil and an anodized film appearing on the surface of thealuminum foil, and a cathode foil made of the aluminum foil, via arelease paper, so that surfaces of both foils face each other;

an electrolytic solution of the present invention;

a case or casing containing the capacitor element and the electrolyticsolution; and

an elastic sealant with which an opening portion of the case is sealed.

In the electrolytic capacitor of the present invention, because theelectrolytic solution of the present invention is used, the functions ofimproving low-temperature stability based on a mixed solvent of anorganic solvent and water, the hydrogen gas absorption function based onaddition of a nitro compound, and prolongation of a working life andreduction of impedance based on inhibition of the hydration reaction dueto use of a specific electrolyte, can be attained.

The electrolytic capacitor of the present invention is preferably formedin such a manner that a capacitor element is formed by winding an anodefoil, wherein the surface of an etched aluminum foil is anodized, and acathode foil made of the etched aluminum foil, via a release paper, sothat surfaces of both foils face each other, and an electrolyticsolution of the present invention are contained in a case, and anopening portion of the case containing the capacitor element is sealedwith an elastic sealant.

FIG. 1 is a sectional view showing one preferred embodiment of theelectrolytic capacitor of the present invention, and FIG. 2 is aperspective view, enlarged partially in the thickness direction, whichshows a capacitor element of the electrolytic capacitor shown in FIG. 1.Although the embodiment shown in the drawings is an electrolyticcapacitor with a wound structure, various changes and modifications maybe made in the electrolytic capacitor of the present invention withoutdeparting from the spirit and scope thereof. Of course, electrolyticcapacitors other than the electrolytic capacitor with a wound structureare included in the scope of the present invention.

The illustrated electrolytic capacitor 10 is a chip-shaped aluminumelectrolytic capacitor and has such a structure that a capacitor element1 impregnated with an electrolytic solution is contained in a metal case4 and an opening portion of the case 4 is sealed with a sealant 3. Thecapacitor element 1 contained in the metal case is in the form of awound sheet-like laminate 20. The laminate 20 comprises, as shown in thedrawing, an aluminum foil (anode) 21 having an oxide film 22 over theentire surface thereof, an aluminum foil (cathode) 23, a first separator(release paper) 24 interposed between these electrodes, and a secondseparator (release paper) 25. The first separator 24 and the secondseparator 25 may be the same or different, but are preferably the samein view of the production process and cost. The second separator 25 maybe formed from a usual insulating film, if it is necessary. Thecapacitor element 1 is impregnated with an electrolytic solution.

In the illustrated electrolytic capacitor 10, the sealant 3 has a leadwire-penetrating hole for inserting a lead wire 2, thereby to conductsealing, therein. The end of the opening portion of the case 4 isprovided with a curl 14 to enhance a sealing strength of the sealant.

The electrolytic capacitor shown in FIGS. 1 and 2 can be produced, forexample, by the following procedure. First, an anode foil, wherein anoxide film is provided over the entire surface, by anodizing thesurface, of a high-purity aluminum foil as a raw material, and a cathodefilm whose surface area is increased by etching the surface are made.Then, the resulting anode foil and cathode foil are disposed facing eachother and a separator (release paper) is interposed between those filmsto form a laminate, thereby making an element with a structure obtainedby winding this laminate, that is, a capacitor element. Subsequently,the resulting capacitor element is impregnated with an electrolyticsolution and the capacitor element impregnated with the electrolyticsolution is contained in a case (generally made of aluminum), asdescribed above, and then an opening portion of the case is sealed witha sealant. Two lead wires are inserted into a lead wire-penetrating holeof the sealant, thereby to completely prevent leakage of theelectrolytic solution.

The electrolytic capacitor of the present invention will be described inmore detail hereinafter.

The aluminum foil used as the anode foil and cathode foil is preferablyan aluminum foil having high purity of 99% or more. The anode foil canbe preferably formed by electrochemically etching the aluminum foil,anodizing it to form an oxide film on the surface, and attaching a leadtab for connecting an electrode. The cathode film can be formed byetching the aluminum foil and attaching a lead tab for connecting anelectrode. This cathode foil may not be anodized.

The capacitor element can be obtained by winding the anode and cathodefoils, formed in the above steps, via the above-described release paperwhile the surfaces of both foils face each other.

The release paper used in the production of the capacitor element is notspecifically limited, but is preferably a paper produced by using as araw material a naturally produced cellulose material, for example,Manila hemp and raw pulp. As the release paper, for example, there canbe advantageously used a paper produced by passing the raw pulp througha dust removing process, a washing process, a beating process andpaper-making process. A paper derived from synthetic fibers can also beused, however, such a paper is not preferred because it is inferior inheat resistance and corrosion of the capacitor is caused by halogen ionscontained in the paper.

The sealant used in the electrolytic capacitor of the present inventioncan be formed from various materials used usually as far as the materialhas high hardness and proper rubber elasticity, and it is alsoimpermeable to an electrolytic solution and has good airtightness forthe sealant. Preferred sealant material includes, for example, elasticrubber such as natural rubber (NR), styrene-butadiene rubber (SBR),ethylene-propylene terpolymer (EPT), and isobutylene-isoprene rubber(IIR). The isobutylene-isoprene rubber (IIR) is preferably used becausethe airtightness is high and the electrolytic solution does notpenetrate in the form of vapor. Vulcanized IIR having more excellentheat resistance, for example, sulfur-vulcanized, quinoid-vulcanized orresin-vulcanized IIR is used more preferably, and the resin-vulcanizedIIR is particularly preferred.

In the practice of the present invention, a hybrid material obtained bylaminating a resin material plate having sufficient airtightness andstrength (e.g. fluorine-contained resin plate such as PTFE plate) can beadvantageously used in place of the above-described sealant material.

EXAMPLES

The following Examples further illustrate the present invention indetail. Note that these examples are to be construed in all respects asillustrative and not restrictive.

Example 1

An aluminum electrolytic capacitor with a wound structure was producedin accordance with the following procedure.

First, an aluminum foil was electrochemically etched, followed byanodizing to form an oxide film over the entire surface of the aluminumfoil, and then a lead tab for connecting an electrode was attached tomake an aluminum anode electrode. Another aluminum foil was alsoelectrochemically etched and a lead tab for connecting an electrode wasattached to make an aluminum cathode electode. Subsequently, a capacitorelement was made by interposing a separator (release paper) between theanode foil and the cathode foil, followed by winding. The capacitorelement was impregnated with an electrolytic solution whose compositionsare shown in Table 1 below and contained in an aluminum case with a baseso that the lead tab for connecting an electrode protrudes out of thecase, and then an opening of this case was sealed with an elasticsealant to make an electrolytic capacitor with a wound structure (10WV-1000 μF).

The specific resistance at 30° C. of the electrolytic solution used inthis example was measured to obtain measured values as described inTable 1 below. After an impedance at low temperature (−40° C.) and animpedance at normal temperature (20° C.) of the electrolytic capacitorthus obtained were measured, an impedance ratio (ratio Z) expressed as aratio of the respective measured values was determined at differentfrequencies: 120 Hz and 100 kHz. As a result, measured values asdescribed in Table 1 below were obtained. To evaluate characteristics ofworking life of the respective electrolytic capacitor, an initial value(characteristic value immediately after production of a capacitor) and acharacteristic value after the capacitor was allowed to stand at hightemperature (lapse of 1000 hours at 105° C.) under application of arated voltage were measured with respect to the capacitance, tan δ andleakage current. As a result, measured values as described in Table 1below were obtained.

Examples 2 to 10

The same procedure as in Example 1 was repeated, except that in thisexample, compositions of the electrolytic solution were changed asdescribed in Table 1 below. The results of characteristic tests aresummarized in Table 1 below.

Comparative Examples 1 to 3

The same procedure as in Example 1 was repeated, except that in thisexample, for the comparison purpose, a nitro compound was eliminatedfrom the electrolytic solution and that compositions of the electrolyticsolution were changed as described in Table 1 below. The results ofcharacteristic tests are summarized in Table 1 below.

TABLE 1 Specific Initial value After 3000 hours at 105° C. Compositionsof resistance Ratio Z Capaci- Leakage Capaci- Leakage Exampleelectrolytic solution at 30° C. 120 Hz 100 kHz tance tan δ current tancetan δ current No. (% by weight) [Ω · cm] [−40/20° C.] [−40/20° C.] [μF][%] [μA] [μF] [%] [μA] Appearance Example Ethylene glycol 25.0 28 1.14.6 1044 5.4 7.7 898 6.2 2.3 1 Water 68.0 Ammonium formate 4.6Hypophosphorous 0.4 acid n-nitroacetophenone 1.0 Nitrobenzoic acid 1.0Example Ethylene glycol 20.0 26 1.1 4.4 1034 5.4 7.2 900 6.2 2.3 2 Water60.0 Ammonium glutarate 16.4 Sulfamic acid 1.6 Nitrophenol 1.0n-nitroacetophenone 1.0 Example Ethylene glycol 15.0 23 1.1 3.9 1025 5.37.0 902 6.1 2.0 3 Water 60.0 Ammonium adipate 23.0 Nitrobenzoic acid 1.0Nitrophenol 1.0 Example Ethylene glycol 22.0 21 1.1 3.8 1020 5.2 6.8 9186.0 2.0 4 Water 50.0 Ammonium succinate 25.0 Benzenesulfonic 0.4 acidNitrophenol 2.6 Example Ethylene glycol 48.0 161 1.2 5.6 1024 8.7 6.2932 9.5 1.9 5 Water 40.0 Ammonium borate 11.0 Nitrobenzoic acid 1.0Example Ethylene glycol 54.0 29 1.0 3.7 1010 5.4 6.1 929 6.2 2.0 6 Water30.0 Ammonium sulfamate 14.6 Phosphoric acid 0.4 n-nitroacetophenone 1.0Example Ethylene glycol 60.0 58 1.0 3.6 1003 6.2 6.3 933 7.0 2.1 7 Water20.0 Ammonium adipate 18.6 Boric acid 0.4 Nitrobenzoic acid 1.0 ExampleEthylene glycol 62.0 94 1.0 3.8 1005 7.1 6.2 940 7.9 2.1 8 Water 27.0Ammonium adipate 9.0 Nitroacetophenone 1.0 Nitrophenol 1.0 ExampleEthylene glycol 40.0 40 1.0 3.6 1018 5.8 6.4 937 6.4 2.2 9 Water 40.0Ammonium glutarate 19.0 Nitrobenzoic acid 1.0 Example Ethylene glycol50.0 68 1.0 3.7 1013 6.4 6.4 942 7.0 2.3 10 Water 39.4 Ammonium adipate9.2 Sulfamic acid 0.4 Nitrophenol 1.0 Comp. Ethylene glycol 60.0 85 1.336.1 1000 7.0 6.5 In all samples, operation of safety- Example Water30.0 vent was caused by gas evolution 1 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution2 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution 3 Ammonium adipate 20.0 within 250hours

As is apparent from the results described in Table 1, the resistivity ofthe electrolytic solution except for Example 5 is almost the same asthat of the Comparative Examples and the specific resistance is smallerthan that of a conventional electrolytic solution. Although the specificresistance of the electrolytic solution of Example 5 shows a large valuesuch as 161 Ω·cm, the electrolytic capacitor is substantially comparablewith a conventional electrolytic capacitor and is suited for practicaluse when generally judged considering other characteristics.Accordingly, the electrolytic capacitor made by using the electrolyticsolution of the present invention can realize a lower impedance than aconventional electrolytic capacitor, or can realize a low impedancewhich is equivalent to that of a conventional one.

It has been found that the electrolytic capacitor using the electrolyticsolution of the present invention has a small ratio Z and that the ratioZ at a high frequency such as 100 kHz is particularly reduced ascompared with those of the Comparative Examples. This fact shows thatthe electrolytic capacitor using the electrolytic solution of thepresent invention exhibits good low-temperature stability over a widefrequency range.

Particularly, the electrolytic capacitor using the electrolytic solutionof the present invention shows stable characteristics under applicationof a rated voltage even after it was allowed to stand at hightemperature (lapse of 3000 hours at 105° C.) by adding the nitrocompound in the electrolytic solution in the amount ranging from 0.01 to3% by weight, and the capacitor itself was not broken by gas generation.On the other hand, it became impossible to use all electrolyticcapacitors of the Comparative Examples using the electrolytic solutioncontaining no nitro compound because a safety-vent operated as a resultof expansion of the case caused by hydrogen gas generation at an initialstage before a lapse of 3000 hours. This fact shows that a working lifeof the electrolytic capacitor can be easily prolonged according to thepresent invention.

Examples 11 to 19

The same procedure as in Example 1 was repeated, except that in thisexample compositions of the electrolytic solution were changed asdescribed in Table 2 below to confirm the effect of simultaneousaddition of a chelate compound and a nitro compound. As is summarized inTable 2 below, satisfactory results could be obtained. In Table 2 below,the test results of Comparative Examples 1 to 3 are also described.

TABLE 2 Specific Initial value After 1000 hours at 105° C. Compositionsof resistance Ratio Z Capaci- Leakage Capaci- Leakage Exampleelectrolytic solution at 30° C. 120 Hz 100 kHz tance tan δ current tancetan δ current No. (% by weight) [Ω · cm] [−40/20° C.] [−40/20° C.] [μF][%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.3 36.11008 7.0 6.5 In all samples, operation of Example Water 30.0 safety-ventwas caused by gas  1 Ammonium adipate 10.0 evolution within 500 hoursComp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of Example Water 40.0 safety-vent was caused by gas  2Ammonium adipate 15.0 evolution within 250 hours Comp. Ethylene glycol30.0 20 1.0 7.9 1023 4.7 6.9 In all samples, operation of Example Water50.0 safety-vent was caused by gas  3 Ammonium adipate 20.0 evolutionwithin 250 hours Example Ethylene glycol 25.0 21 1.1 4.6 1044 5.2 7.8919 5.8 2.5 satisfactory 11 Water 69.4 Ammonium formate 4.0Hypophophorous 0.4 acid p-nitrobenzoic 0.8 acid EDTA 0.4 ExampleEthylene glycol 20.0 26 1.1 4.4 1036 5.4 7.3 922 6.0 2.3 satisfactory 12Water 59.2 Ammonium glurarate 17.8 Sulfamic acid 1.6 Nitrophenol 1.0EDTA 0.4 Example Ethylene glycol 15.0 23 1.1 3.9 1028 5.3 7.1 925 5.92.2 satisfactory 13 Water 58.7 Ammonium adipate 24.4 Dinitrobenzoic acid1.5 EDDA 0.4 Example Ethylene glycol 24.2 21 1.1 3.8 1021 5.2 6.9 9305.8 2.2 satisfactory 14 Water 50.0 Ammonium succinate 24.2Benzenesulfonic 0.4 acid p-nitrobenzoic acid 0.8 DTPA 0.4 ExampleEthylene glycol 55.0 29 1.0 3.7 1009 5.4 6.2 938 6.0 2.2 satisfactory 15Water 28.0 Ammonium sulfamate 14.0 Phosphoric acid 2.0 Nitrophenol 0.6EDTA 0.4 Example Ethylene glycol 59.2 57 1.0 3.6 1002 6.1 6.4 944 6.72.4 satisfactory 16 Water 20.0 Ammonium adipate 19.0 Boric acid 0.4Nitroacetophene 1.0 EDDA 0.4 Example Ethylene glycol 62.0 92 1.0 3.81003 7.0 6.5 942 7.6 2.4 satisfactory 17 Water 27.0 Ammonium adipate 9.3EDTPO 0.1 Nitrobenzoic acid 1.2 EDTA 0.4 Example Ethylene glycol 38.8 391.0 3.6 1018 5.8 6.3 937 6.4 2.1 satisfactory 18 Water 40.0 Ammoniumglutarate 19.6 Nitrophenol 1.2 EDTPO 0.4 Example Ethylene glycol 48.8 681.0 3.7 1014 6.4 6.2 943 7.0 2.2 satisfactory 19 Water 40.0 Ammoniumadipate 9.2 Sulfamic acid 0.4 Nitrophenol 1.2 EDDA 0.4

Examples 20 to 29

The same procedure as in Example 1 was repeated, except that in thisexample compositions of the electrolytic solution were changed asdescribed in Table 3 below to confirm the effect of simultaneousaddition of saccharides and a nitro compound. As is summarized in Table3 below, satisfactory results could be obtained. In Table 3 below, thetest results of Comparative Examples 1 to 3 are also described.

TABLE 3 Specific Initial value After 1000 hours at 105° C. Compositionsof resistance Ratio Z Capaci- Leakage Capaci- Leakage Exampleelectrolytic solution at 30° C. 120 Hz 100 kHz tance tan δ current tancetan δ current No. (% by weight) [Ω · cm] [−40/20° C.] [−40/20° C.] [μF][%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.3 36.11008 7.0 6.5 In all samples, operation of safety- Example Water 30.0vent was caused by gas evolution  1 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution 2 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution  3 Ammonium adipate 20.0 within250 hours Example Ethylene glycol 23.2 22 1.1 4.6 1043 5.3 7.7 918 5.92.5 satisfactory 20 Water 70.0 Ammonium formate 4.4 Galactose 1.0Nitrophenol 1.0 Hypophosphorous acid 0.4 Example Ethylene glycol 19.4 271.1 4.4 1035 5.4 7.2 921 6.0 2.3 satisfactory 21 Water 60.0 Ammoniumglutarate 17.8 Fluctose 1.0 Nitrobenzoic acid 0.8 Sulfamic acid 1.0Example Ethylene glycol 14.2 24 1.1 3.9 1027 5.3 7.0 924 5.9 2.2satisfactory 22 Water 60.0 Ammonium adipate 23.8 Dinitrobenzoic acid 1.0Xylose 1.0 Example Ethylene glycol 20.8 22 1.1 3.8 1020 5.3 6.8 930 5.92.2 satisfactory 23 Water 50.0 Ammonium succinate 24.8 Glucose 1.0Nitrophenol 3.0 Benzenesulfonic acid 0.4 Example Ethylene glycol 48.7162 1.2 5.6 1014 8.8 6.2 933 9.4 2.1 satisfactory 24 Water 39.8 Ammoniumborate 9.0 Nitrophenol 1.5 Xylose 1.0 Example Ethylene glycol 53.2 301.0 3.7 1008 5.5 6.1 937 6.1 2.2 satisfactory 25 Water 30.0 Ammoniumsulfamate 13.8 Fluctose 1.0 Nitrobenzoic acid 1.5 Phosphoric acid 0.5Example Ethylene glycol 59.2 59 1.0 3.6 1001 6.2 6.3 944 6.8 2.4satisfactory 26 Water 20.0 Ammonium adipate 17.8 Glucose 1.0Dinitrobenzoic acid 1.0 Boric acid 1.0 Example Ethylene glycol 60.9 911.0 3.8 1002 7.0 6.2 944 7.6 2.4 satisfactory 27 Water 28.0 Ammoniumadipate 9.3 Nitrobenzoic acid 0.8 Fluctose 1.0 Example Ethylene glycol38.2 40 1.0 3.6 1018 5.8 6.3 937 6.4 2.1 satisfactory 28 Water 40.0Ammoniun glutarate 18.8 Nitroacetophenone 2.0 Galactose 1.0 ExampleEthylene glycol 47.7 69 1.0 3.7 1013 6.4 6.2 942 7.0 2.2 satisfactory 29Water 39.4 Ammonium adipate 9.0 Sulfamic acid 0.4 Nitrobenzoic acid 2.5Xylose 1.0

Examples 30 to 39

The same procedure as in Example 1 was repeated, except that in thisexample compositions of the electrolytic solution were changed asdescribed in Table 4 below to confirm the effect of simultaneousaddition of hydroxybenzyl alcohol, glutamic-diacetic acid and the likeas well as a nitro compound. As is summarized in Table 4 below,satisfactory results could be obtained. In Table 4 below, the testresults of Comparative Examples 1 to 3 are also described.

TABLE 4 Specific Ratio Z Initial value After 1000 hours at 105° C.Compositions of resistance 120 Hz 100 kHz Capac- Leakage Leakage Exampleelectrolytic solution at 30° C. [−40/ [−40/ itance tan δ currentCapacitance tan δ current No. (% by weight) [Ω · cm] 20° C.] 20° C.][μF] [%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.336.1  1008 7.0 6.5 In all samples, operation of safety- Example Water30.0 vent was caused by gas evolution 1 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution2 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution 3 Ammonium adipate 20.0 within 250hours Example Ethylene glycol 24.0 21 1.1 4.6 1044 5.2 7.7 919 5.8 2.5satisfactory 30 Water 68.0 Ammonium formate  4.4 Hypophosphorous  0.4acid Dinitrobenzoic  1.2 acid Hydroxybenzyl  2.0 alcohol ExampleEthylene glycol 17.7 27 1.1 4.4 1034 5.4 7.2 920 6.0 2.3 satisfactory 31Water 60.0 Ammonium 16.8 glutarate Sulfamic acid  1.6 Nitrophenol  2.5Glutamic-diacetic  1.4 acid Example Ethylene glycol 14.2 24 1.1 3.9 10255.3 7.0 923 5.9 2.2 satisfactory 32 Water 60.0 Ammonium adipate 23.4Nitrobenzoic acid  1.0 Glutamic-diacetic  1.4 acid Example Ethyleneglycol 20.8 22 1.1 3.8 1020 5.3 6.8 930 5.9 2.2 satisfactory 33 Water50.0 Ammonium 24.8 succinate Benzosulfonic  0.4 acid Nitrobenzoic acid 2.0 Hydrobenzyl  2.0 alcohol Example Ethylene glycol 44.7 161  1.2 5.61024 8.7 6.2 942 9.3 2.1 satisfactory 34 Water 40.0 Ammonium borate  9.8Dinitrobenzoic  1.5 acid Hydroxybenzyl  4.0 alcohol Example Ethyleneglycol 52.2 30 1.0 3.7 1010 5.5 6.1 939 6.1 2.2 satisfactory 35 Water30.0 Ammonium 13.8 sulfamate Phosphoric acid  0.4 Nitroacetophenone  1.0Glutamic-diacetic  2.6 acid Example Ethylene glycol 57.2 63 1.0 3.6 10036.3 6.3 944 6.9 2.4 satisfactory 36 Water 20.0 Ammonium adipate 15.0Nitrophenol  3.0 Hydroxybenzyl  2.6 alcohol Glutamic-diacetic  2.2 acidExample Ethylene glycol 59.2 95 1.0 3.8 1005 7.1 6.2 944 7.7 2.4satisfactory 37 Water 27.0 Ammonium adipate  9.3 Dinitrobenzoic  1.5acid Ammonium  1.5 glutarate Glutamic-diacetic  1.5 acid ExampleEthylene glycol 39.6 39 1.0 3.6 1018 5.8 6.3 937 6.4 2.1 satisfactory 38Water 40.0 Ammonium 19.0 glutarate Nitrophenol  0.4 Hydroxybenzyl  1.0alcohol Example Ethylene glycol 48.7 68 1.0 3.7 1013 6.4 6.4 942 7.0 2.3satisfactory 39 Water 39.4 Ammonium adipate  9.0 Sulfamic acid  0.4Dinitrobenzoic  1.5 acid Hydroxybenzyl  1.0 alcohol

Examples 40 to 49

The same procedure as in Example 1 was repeated, except that in thisexample compositions of the electrolytic solution were changed asdescribed in Table 5 below to confirm the effect of simultaneousaddition of a nitro compound and gluconic lactone. As is summarized inTable 5 below, satisfactory results could be obtained. In Table 5 below,the test results of Comparative Examples 1 to 3 are also described.

TABLE 5 Specific Ratio Z Initial value After 3000 hours at 105° C.Compositions of resistance 120 Hz 100 kHz Capac- Leakage Leakage Exampleelectrolytic solution at 30° C. [−40/ [−40/ itance tan δ currentCapacitance tan δ current No. (% by weight) [Ω · cm] 20° C.] 20° C.][μF] [%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.336.1  1008 7.0 6.5 In all samples, operation of safety- Example Water30.0 vent was caused by gas evolution 1 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution2 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution 3 Ammonium adipate 20.0 within 250hours Example Ethylene glycol 25.0 28 1.1 4.6 1044 5.6 7.7 898 6.4 2.3satisfactory 40 Water 68.0 Ammonium formate  4.4 Hypophosphorous  0.4acid Gluconic lactone  0.2 n-nitroacetophenone  1.0 n-nitrobenzoic acid 1.0 Example Ethylene glycol 20.0 26 1.1 4.4 1034 5.5 7.2 900 6.3 2.3satisfactory 41 Water 60.0 Ammonium 16.2 glutarate Sulfamic acid  1.6Gluconic lactone  0.2 Nitrophenol  1.0 n-nitroacetophenone  1.0 ExampleEthylene glycol 15.0 23 1.1 3.9 1025 5.4 7.0 902 6.2 2.0 satisfactory 42Water 60.0 Ammonium adipate 22.8 Gluconic lactone  0.2 Nitrobenzoic acid 1.0 Nitrophenol  1.0 Example Ethylene glycol 22.0 21 1.1 3.8 1020 5.26.8 918 6.0 2.0 satisfactory 43 Water 50.0 Ammonium 25.0 succinateBenzenesulfonic  0.4 acid Gluconic lactone  0.2 Nitrophenol  2.6 ExampleEthylene glycol 48.0 161  1.2 5.6 1024 8.8 6.2 932 9.6 1.9 satisfactory44 Water 40.0 Ammonium borate 10.8 Gluconic lactone  0.2 Nitrobenzoicacid  1.0 Example Ethylene glycol 54.0 29 1.0 3.7 1010 5.6 6.1 929 6.42.0 satisfactory 45 Water 30.0 Ammonium 14.4 sulfamate Phosphoric acid 0.4 Gluconic lactone  0.2 n-nitroacetophenone  1.0 Example Ethyleneglycol 60.0 58 1.0 3.6 1003 6.2 6.3 933 7.0 2.1 satisfactory 46 Water20.0 Ammonium adipate 16.4 Boric acid  0.4 Gluconic lactone  0.2Nitrobenzoic acid  1.0 Example Ethylene glycol 62.0 95 1.0 3.8 1005 6.96.2 940 7.7 2.1 satisfactory 47 Water 27.0 Ammonium adipate  6.8Gluconic lactone  0.2 n-nitroacetophenone  1.0 Nitrophenol  1.0 ExampleEthylene glycol 40.0 40 1.0 3.6 1018 5.8 6.3 937 6.4 2.2 satisfactory 48Water 40.0 Ammonium 18.8 glutarate Gluconic lactone  0.2 Nitrobenzoicacid  1.0 Example Ethylene glycol 50.0 68 1.0 3.7 1013 6.5 6.4 942 7.12.3 satisfactory 49 Water 39.4 Ammonium adipate  9.0 Sulfamic acid  0.4Gluconic lactone  0.2 Nitrophenol  1.0

Examples 50 to 59

The same procedure as in Example 1 was repeated, except that in thisexample compositions of the electrolytic solution were changed asdescribed in Table 6 below to confirm the effect obtained by anarbitrary combination of various additives. As is summarized in Table 6below, satisfactory results could be obtained. In Table 6 below, thetest results of Comparative Examples 1 to 3 are also described.

TABLE 6 Specific Ratio Z Initial value After 3000 hours at 105° C.Compositions of resistance 120 Hz 100 kHz Capac- Leakage Leakage Exampleelectrolytic solution at 30° C. [−40/ [−40/ itance tan δ currentCapacitance tan δ current No. (% by weight) [Ω · cm] 20° C.] 20° C.][μF] [%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.336.1  1008 7.0 6.5 In all samples, operation of safety- Example Water30.0 vent was caused by gas evolution 1 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution2 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution 3 Ammonium adipate 20.0 within 250hours Example Ethylene glycol 24.0 28 1.1 4.6 1044 5.3 7.7 898 6.1 2.3satisfactory 50 Water 68.0 Ammonium formate  4.4 Hypophosphorous  0.4acid EDTA  0.5 Glutamic-diacetic  0.2 acid Gluconic lactone  0.2n-nitroacetophenone  1.0 Nitrobenzoic acid  1.0 Example Ethylene glycol18.0 26 1.1 4.4 1034 5.2 7.2 900 6.0 2.3 satisfactory 51 Water 60.0Ammonium 16.2 glutarate Sulfamic acid  1.6 DTPA  1.0 Fluctose  1.0Gluconic lactone  0.2 Nitrophenol  1.0 n-nitroacetophenone  1.0 ExampleEthylene glycol 15.0 23 1.1 3.9 1025 5.5 7.0 902 6.3 2.0 satisfactory 52Water 57.5 Ammonium adipate 22.8 EDTA  2.0 Hydroxybenzyl  0.5 alcoholGluconic lactone  0.2 Nitrobenzoic acid  1.0 Nitrophenol  1.0 ExampleEthylene glycol 20.6 21 1.1 3.8 1020 5.4 6.8 918 6.2 2.0 satisfactory 53Water 50.0 Ammonium 25.0 succinate EDDA  1.0 Glutamic-diacetic  0.2 acidBenzenesulfonic  0.4 acid Gluconic lactone  0.2 Nitrophenol  2.6 ExampleEthylene glycol 46.7 161  1.2 5.6 1024 8.9 6.2 932 9.7 1.9 satisfactory54 Water 40.0 Ammonium borate 10.8 EDDA  0.8 Hydroxybenzyl  0.5 alcoholGluconic lactone  0.2 Nitrobenzoic acid  1.0 Example Ethylene glycol52.0 29 1.0 3.7 1010 5.2 6.1 929 6.0 2.0 satisfactory 55 Water 30.0Ammonium 14.4 sulfamate Xylose  0.5 EDTPO  1.0 Glutamic-diacetic  0.5acid Phosphoric acid  0.4 Gluconic lactone  0.2 n-nitroacetophenone  1.0Example Ethylene glycol 57.5 58 1.0 3.6 1003 6.1 6.3 933 6.9 2.1satisfactory 56 Water 20.0 Ammonium adipate 18.4 EDTA  1.5 Hydroxybenzyl 0.5 alcohol Glutamic-diacetic  0.5 acid Boric acid  0.4 Gluconiclactone  0.2 Nitrobenzoic acid  1.0 Example Ethylene glycol 57.5 95 1.03.8 1005 6.0 6.2 940 7.6 2.1 satisfactory 57 Water 27.0 Ammonium adipate 8.8 DTPA  3.0 Glucose  1.0 Hydroxybenzyl  6.5 alcohol Gluconic lactone 0.2 n-nitroacetophenone  1.0 Nitrophenol  1.0 Example Ethylene glycol37.9 40 1.0 3.6 1018 4.8 6.4 937 5.4 2.2 satisfactory 58 Water 40.0 EDDA 1.0 Fluctose  0.5 Glutamic-diacetic  0.6 acid Ammonium 18.8 glutarateGluconic lactone  0.2 Nitrobenzoic acid  1.0 Example Ethylene glycol47.5 68 1.0 3.7 1013 6.8 6.4 942 7.4 2.3 satisfactory 59 Water 39.4Ammonium adipate  9.0 Hydroxybenzyl  1.0 alcohol EDTPO  1.0 Fluctose 0.5 Sulfamic acid  0.4 Gluconic lactone  0.2 Nitrophenol  1.0

Comparative Examples 4 to 6 and Examples 60 to 62

The same procedure as in Example 1 was repeated, except that in thisexample the measurement of the characteristic value under hightemperature conditions (application of rated voltage, lapse of 1000hours at 105° C.) employed in Example 1 was conducted under conditions(lapse of 6000 hours at 105° C.) to confirm a further improvement incharacteristics of working life. The results as described in Table 7below were obtained.

TABLE 7 Specific Ratio Z Initial value After 6000 hours at 105° C.Compositions of resistance 120 Hz 100 kHz Capac- Leakage Leakage Exampleelectrolytic solution at 30° C. [−40/ [−40/ itance tan δ currentCapacitance tan δ current No. (% by weight) [Ω · cm] 20° C.] 20° C.][μF] [%] [μA] [μF] [%] [μA] Appearance Comp. Ethylene glycol 60.0 85 1.336.1  1008 7.0 6.5 In all samples, operation of safety- Example Water30.0 vent was caused by gas evolution 4 Ammonium adipate 10.0 within 500hours Comp. Ethylene glycol 45.0 40 1.1 9.7 1014 5.7 6.1 In all samples,operation of safety- Example Water 40.0 vent was caused by gas evolution5 Ammonium adipate 15.0 within 250 hours Comp. Ethylene glycol 30.0 201.0 7.9 1023 4.7 6.9 In all samples, operation of safety- Example Water50.0 vent was caused by gas evolution 6 Ammonium adipate 20.0 within 250hours Example Ethylene glycol 25.0 28 1.1 4.6 1044 5.4 7.7 855 6.6 2.1satisfactory 60 Water 68.0 Ammonium formate  4.6 Hypophosphorous  0.4acid n-nitroacetophenone  1.0 Nitrobenzoic acid  1.0 Example Ethyleneglycol 15.0 23 1.1 3.9 1025 5.3 7.0 668 8.2 1.6 satisfactory 61 Water60.0 Ammonium adipate 23.0 Nitrobenzoic acid  1.0 Nitrophenol  1.0Example Ethylene glycol 40.0 40 1.0 3.6 1018 5.8 6.4 632 9.1 1.1satisfactory 62 Water 40.0 Ammonium 19.0 glutarate Nitrobenzoic acid 1.0

In Table 7, Comparative Examples 4 to 6 respectively correspond toComparative Examples 1 to 3, while Examples 60 to 62 respectivelycorrespond to Examples 1, 3 and 9. As is apparent from the results, itbecomes impossible to use all capacitors of Comparative Examples 4 to 6using an electrolytic solution having added thereto no nitro compound,whereas, capacitors of Examples 60 to 62 could be used even after 6000hours had passed although a reduction in capacitance was recognized.Surprisingly, it has been found that characteristics of working life ofthe electrolytic capacitor are further improved by using a carboxylicacid or a salt thereof as an organic electrolyte in combination with aninorganic acid as an inorganic electrolyte.

Industrial Applicability

As described above, according to the present invention, there isprovided an electrolytic solution, for use in an electrolytic capacitor,which has a low impedance and excellent low-temperature stabilityexpressed by an a ratio of an impedance at low temperature to that atnormal temperature, along with good characteristics of working life, andalso it can exhibit an excellent hydrogen gas absorption function whenan electrolytic solution contains a highly increased amount of water inits mixed solvent or when an electrolytic capacitor is used under hightemperature conditions. According to the present invention, there isalso provided an electrolytic capacitor with high reliability, which hasa low impedance and excellent low-temperature stability, along with goodcharacteristics of working life, and also it is free from defects due topresence of water used as a solvent, specially an aluminum electrolyticcapacitor.

What is claimed is:
 1. An electrolytic solution for use in anelectrolytic capacitor, comprising a solvent consisting of 20 to 80% byweight of an organic solvent and 80 to 20% by weight of water, and atleast one electrolyte selected from the group consisting of a carboxylicacid or a salt thereof and an inorganic acid or a salt thereof, havingadded thereto at least one nitro compound selected from the groupconsisting of nitrophenol, nitrobenzoic acid, dinitrobenzoic acid,nitroacetophenone and nitroanisole, said electrolytic solution beingcontained between a wound anode foil and a wound cathode foil opposed tothe anode foil of said electrolytic capacitor.
 2. The electrolyticsolution for use in an electrolytic capacitor according to claim 1,wherein the nitro compound is a combination of two or more nitrocompounds.
 3. The electrolytic solution for use in an electrolyticcapacitor according to claim 1, wherein the nitro compound is added inthe amount of 0.01 to 5% by weight based on the total amount of theelectrolytic solution.
 4. The electrolytic solution for use in anelectrolytic capacitor according to claim 1, wherein the organic solventis a protic solvent, an aprotic solvent, or a mixture thereof.
 5. Theelectrolytic solution for use in an electrolytic capacitor according toclaim 1, wherein the carboxylic acid or salt thereof is selected fromthe group consisting of formic acid, acetic acid, propionic acid,butyric acid, p-nitrobenzoic acid, salicylic acid, benzoic acid, oxalicacid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaricacid, maleic acid, phthalic acid, azelaic acid, citric acid andhydroxybutyric acid, as well as ammonium, sodium, potassium, amine andalkyl ammonium salts thereof.
 6. The electrolytic solution for use in anelectrolytic capacitor according to claim 1, wherein the inorganic acidor salt thereof is selected from the group consisting of phosphoricacid, phosophorous acid, hypophosphorous acid, boric acid, sulfamicacid, as well as ammonium, sodium, potassium, amine and alkyl ammoniumsalts thereof.
 7. The electrolytic solution for use in an electrolyticcapacitor according to claim 1, further comprising at least one additiveselected from the group consisting of the following group: (1) a chelatecompound, (2) saccharides, (3) hydroxybenzyl alcohol and/orL-glutamic-diacetic acid or a salt thereof, and (4) gluconic acid and/orgluconic lactone.
 8. An electrolytic capacitor comprising a capacitorelement formed of a wound anode foil, a wound cathode foil opposed tothe anode foil and a separator disposed between the anode foil and thecathode foil, and a electrolytic solution contained between the anodefoil and the cathode foil, said electrolytic solution containing asolvent consisting of 20 to 80% by weight of an organic solvent and 80to 20% by weight of water, and at least one electrolyte selected fromthe group consisting of a carboxylic acid or a salt thereof and aninorganic acid or a salt thereof, having added thereto at least onenitro compound selected from the group consisting of nitrophenol,nitrobenzoic acid, dinitrobenzoic acid, nitroacetophenone andnitroanisole.
 9. The electrolytic capacitor according to claim 8,wherein the nitro compound is a combination of two or more nitrocompounds.
 10. The electrolytic capacitor according to claim 8, whereinthe nitro compound is added in the amount of 0.01 to 5% by weight basedon the total amount of the electrolytic solution.